The present invention relates to data communication and, in particular embodiments, to the communication of data over a channel exhibiting intersymbol interference.
An intersymbol interference (ISI) channel is one in which, as a result of distortion in the channel, the signal energy of a signal point transmitted in one signaling interval becomes dispersed over a number of adjacent signaling intervals. The dispersed energy combines with signal points transmitted in the adjacent intervals and thus constitutes a source of noise in those other intervals. When the level of ISI is small, a so-called linear equalizer is effective in mitigating against it. However, if the ISI is severe, other, more powerful techniques must be brought into play. Typically these techniques make use of a decision feedback equalizer (DFE). A DFE estimates the amount of ISI in a given received signal point and subtracts the ISI estimate therefrom to arrive at an ISI-compensated signal point from which a decision is made as to the identity of the transmitted signal point.
One such technique is taught in my co-pending U.S. patent application Ser. No. 09/049268 filed Mar. 3, 1998 and entitled xe2x80x9cPath-Oriented Decoder Using Refined Receiver Trellis Diagram.xe2x80x9d The ISI phenomenon is manifested in that some of the energy of the signal point transmitted during a signaling interval is dispersed into adjacent signaling intervals. The technique described in that patent application uses a so-called path-oriented joint decoder/DFE with a refined receiver trellis diagram to in effect return to each signal point at least a portion of the signal energy thereof that was dispersed to other signaling intervals. This provides an improvement in error immunity referred to as a xe2x80x9cconversion gainxe2x80x9d.
The aforementioned conversion gain is not fully achieved for the last several signal points of any given transmission. The reason for this is that there is a smaller number of subsequent signaling intervals from which the dispersed signal energy of these last signal points can be gathered and returned to the respective signal points. Indeed, that smaller number may be zero relative to the very last signal point. The decoding of these ending signal points is therefore less reliable than that of the signal points for which conversion gain was secured. This problem can be overcome by artificially creating additional signaling intervals through the transmission of dummy signal points, thereby providing an opportunity for the dispersed energy of the actual data signal points to be gathered as discussed above.
This above-described technique is quite workable in a continuous transmission environmentxe2x80x94in which a stream of many millions of bits may be transmittedxe2x80x94because the resulting reduction in bandwidth efficiency (average number of useful data bits per signaling interval) will be negligible. However, for packet transmission, in which the signal points are transmitted in discrete packets each containing a relatively small number of signal points (typically on the order of several hundreds), the introduction of dummy signal points may well have an undesirably large affect on the bandwidth efficiency. On the other hand, to not transmit dummy signal points means that the decoding of the last few signal points will be less reliable than that of the preceding signal points. This is quite disadvantageous in a packet transmission environment because the performance of the ending signal points relative to the other signal points would then dominate the overall error performance, given that, typically, an entire packet is flagged as being in error even if the decoding of only one of its signal points is in error.
The present invention is directed to a technique for use in a data transmission environment in which, as per the discussion above, a portion of the signal points in a signal point stream benefit from a particular form of error immunity enhancementxe2x80x94e.g., the aforementioned conversion gainxe2x80x94while other signal points benefit less therefrom, including not benefiting at all. In accordance with the principles of the invention, signal points appearing at predetermined positions in the signal point stream that benefit less from the error immunity enhancement are transmitted in such a way that their robustness against errors is at least as great as that of the signal points that benefit more therefrom. Thus the invention eliminates the above-described problem of the ending signal points dominating the overall error rate performance of, for example, a packet transmission signaling scheme.
In the disclosed illustrative packet transmission embodiments of the invention, the ending signal points of a packet are transmitted using a signal point constellation that has fewer signal points than is used for those which precede them so that the ending signal points can be further apart from one another in signal space. As a result, more noise may be tolerated and the overall error performance of the ending signal points can be made no worse than that of the other signal points. Thus, essentially the same overall packet error rate as that which is achieved for continuous transmission can be achieved for packet transmission because (a) the signal points that occur earlier in the packet benefit from conversion gain and (b) another form of enhanced error immunity is provided for the ending signal points which do not benefit from conversion gain. Furthermore, the invention significantly ameliorates the reduction in bandwidth efficiency that would be occasioned by the use of dummy signal points as described above.
The invention may be used in signaling schemes which utilize coded modulation such as convolutional coding. When such coding is used in packet transmission environments (or other environments in which the data is transmitted in successive disjoint communications), it is advantageous to terminate the code. This means that the transmitter coder is brought to a predetermined terminating state in order to avoid significant decoding delay in the receiver while yet preserving for all of the signal points in the packet the error immunity afforded by the code. Code-termination alone, however, does not always provide for the above-discussed conversion gain. Therefore, in preferred implementations of coded modulation transmission systems which incorporate the present invention, first the code is terminated and then a number of ending signal points are transmitted pursuant to the principles of the invention as described above.
The principles of the invention may also be used in a continuous transmission environment and are applicable, in particular, to those arrangements in which the path-oriented joint decoder/DFE is employed with modulation schemes utilizing relatively large constellations (e.g., larger than a 16-QAM or 32-QAM). In such arrangements, the ISI components are far stronger than when the constellation has fewer signal points and, as a result, once a decoding error is made, the error propagation is more severe. Therefore, to confine the error propagation, it is desirable to divide the continuous transmission into packets and then to apply the principles of the invention to each packet.